It was easy to find a list of shopping carts here:

http://www.cybersource.com/partners/...rts/solutions/

I'd never heard of them before though, and I would recommend that your client also accepts PayPal at the very least. Whatever cart they choose will support PP as it's one of the first ones that any developer would implement, and some people will want to use PayPal to the exclusion of any other option. Depending on the countries where their customers are located, supporting other forms of payment may be beneficial too.

In addition to searching here, forums such as WHT (webhostingtalk.com) are very good places to find information about payment providers and technologies such as shopping carts. Sitepoint may also be useful.